Module adapton::macros
[−]
[src]
Macros to make using the engine
module's interface more
ergonomic.
Demand-driven change propagation
The example below demonstrates demand-driven change propagation,
which is unique to Adapton's approach to incremental computation. The
example constructs two mutable inputs, nom
and den
, an
intermediate subcomputation div
that divides the numerator in nom
by the denominator in den
, and a thunk check
that first checks
whether the denominator is zero (returning zero if so) and if
non-zero, returns the value of the division.
use adapton::macros::*; use adapton::engine::*; manage::init_dcg(); // Two mutable inputs, for numerator and denominator of division let num = cell!(42); let den = cell!(2); // In Rust, cloning is explicit: let den2 = den.clone(); // clone _global reference_ to cell. let den3 = den.clone(); // clone _global reference_ to cell, again. // Two subcomputations: The division, and a check thunk with a conditional expression let div = thunk![ get!(num) / get!(den) ]; let check = thunk![ if get!(den2) == 0 { None } else { Some(get!(div)) } ]; // Observe output of `check` while we change the input `den` // Step 1: (Explained in detail, below) assert_eq!(get!(check), Some(21)); // Step 2: (Explained in detail, below) set(&den3, 0); assert_eq!(get!(check), None); // Step 3: (Explained in detail, below) set(&den3, 2); assert_eq!(get!(check), Some(21)); // division is reused
The programmer's changes and observations in the last lines induce the following change propagation behavior:
When the
check
is demanded the first time, it executes the condition, andden
holds2
, which is non-zero. Hence, theelse
branch executesget!(div)
, which demands the output of the division,21
.After this first observation of
check
, the programmer changesden
to0
, and re-demands the output ofcheck
. In response, change propagation first re-executes the condition (not the division), and the condition branches to thethen
branch, resulting inNone
; in particular, it does not re-demand thediv
node, though this node still exists in the DCG.Next, the programmer changes
den
back to its original value,2
, and re-demands the output ofcheck
. In response, change propagation re-executes the condition, which re-demands the output ofdiv
. Change propagation attempts to "clean" thediv
node before re-executing it. To do so, it compares its last observations ofnum
andden
to their current values, of42
and2
, respectively. In so doing, it finds that these earlier observations match the current values. Consequently, it reuses the output of the division (21
) without having to re-execute the division.
For a graphical illustration of this behavior, see these slides.
In the academic literature on Adapton, we refer to this three-step
pattern as switching: The demand of div
switches from being
present (in step 1) to absent (in step 2) to present (in step 3).
Past work on self-adjusting computation does not support this switching pattern directly: Because of its change propagation semantics, it would "forget" the division in step 2, and rerun it from-scratch in step 3.
Furthermore, some other change propagation algorithms base their re-execution schedule on "node height" (of the graph's topological ordering). These algorithms may also have undesirable behavior. In particular, they may re-execute the division in step 2, though it is not presently in demand. For an example, see this gist.
Use force_map
for finer-grained dependence tracking
Below, we show that using force_map
prunes the dirtying phase of
change propagation; this test traces the engine, counts the number of
dirtying steps, and ensures that this count is zero, as expected.
use adapton::macros::*; use adapton::engine::*; use adapton::reflect; manage::init_dcg(); // Trace the behavior of change propagation; ensure dirtying works as expected reflect::dcg_reflect_begin(); let pair = cell!((1234, 5678)); let pair1 = pair.clone(); let t = thunk![{ // Project the first component of pair: let fst = force_map(&pair, |_,x| x.0); fst + 100 }]; // The output is `1234 + 100` = `1334` assert_eq!(force(&t), 1334); // Update the second component of the pair; the first is still 1234 set(&pair1, (1234, 8765)); // The output is still `1234 + 100` = `1334` assert_eq!(force(&t), 1334); // Assert that nothing was dirtied (due to using `force_map`) let traces = reflect::dcg_reflect_end(); let counts = reflect::trace::trace_count(&traces, None); assert_eq!(counts.dirty.0, 0); assert_eq!(counts.dirty.1, 0);
Nominal memoization: Toy Examples
Adapton offers nominal memoization, which uses first-class names
(each of type Name
) to identify cached computations and data. Behind
the scenes, these names control how and when the engine overwrites
cached data and computations. As such, they permit patterns of
programmatic cache eviction.
For a simple illustration, we memoize several function calls to sum
with different names and arguments. In real applications, the
memoized function typically performs more work than summing two
machine words. :)
use adapton::macros::*; use adapton::engine::*; use adapton::reflect; // create an empty DCG (demanded computation graph) manage::init_dcg(); // a simple function (memoized below for illustration purposes; // probably actually not worth it!) fn sum(x:usize, y:usize) -> usize { x + y } // Optional: Traces what the engine does below (for diagnostics, testing, illustration) reflect::dcg_reflect_begin(); let nm_a_0 : Name = name_of_str("a"); // name "a" let nm_a_1 : Name = name_of_str("a"); // name "a" (another copy) let nm_b_0 : Name = name_of_str("b"); // name "b" let nm_b_1 : Name = name_of_str("b"); // name "b" (another copy) // create a memo entry, named "a", that remembers that `sum(42,43) = 85` let res1 : usize = memo!(nm_a_0 =>> sum, x:42, y:43); // same name "a", same arguments (42, 43) => reuses the memo entry above for `res1` let res2 : usize = memo!(nm_a_1 =>> sum, x:42, y:43); // different name "b", same arguments (42, 43) => will *not* match `res1`; creates a new entry let res3 : usize = memo!(nm_b_0 =>> sum, x:42, y:43); // same name "b", different arguments; will *overwrite* entry named "b" with new args & result let res4 : usize = memo!(nm_b_1 =>> sum, x:55, y:66); // Optional: Assert what happened above, in terms of analytical counts let traces = reflect::dcg_reflect_end(); let counts = reflect::trace::trace_count(&traces, None); // Editor allocated two thunks ("a" and "b") assert_eq!(counts.alloc_fresh.0, 2); // Editor allocated one thunk without changing it ("a", with same args) assert_eq!(counts.alloc_nochange.0, 1); // Editor allocated one thunk by changing it ("b", different args) assert_eq!(counts.alloc_change.0, 1); // Archivist allocated nothing assert_eq!(counts.alloc_fresh.1, 0);
Some notes about the code above:
Callsite argument names: The macro
memo!
relies on programmer-supplied variable names in its macro expansion of these call sites, shown asx
andy
in the uses above. These can be chosen arbitrarily: So long as these symbols are distinct from one another, they can be any symbols, and need not actually match the formal argument names.Type arguments: If the function call expects type arguments,
memo!
accomodates these calls with alternative syntax.Spurious arguments: If the function call expects some later arguments that do not implement
Eq
, but are functionally determined by earlier ones that do (including the suppliedName
),memo!
accomodates these calls with alternative syntax. We call these arguments "spurious", since the Adapton engine does not check their identity when performing change propagation. Common examples include function values (e.g., anonymous closures).
Structs
ProgPt |
Program points: used by the Adapton engine to distinguish different memoized functions. |
Rc |
A single-threaded reference-counting pointer. |
Functions
bump_name_counter |
Convenience function: A global counter for creating unique names, e.g., in unit tests. Avoid using this outside of unit tests. |